The efferent innervation to the cochlea arises from neurons in the brainstem which project to the peripheral auditory receptor, the organ of Corti, where they make synaptic contacts with either cochlear nerve fibers beneath the inner hair cells or with outer hair cells directly. These connections comprise a complex feedback mechanism that can suppress or enhance acoustic reception, depending of the specific efferent subsystem involved. Recent studies in the rat and guinea pig indicate that olivocochlear neurons that contact cochlear nerve fibers beneath the inner hair cells, known as lateral olivocochlear neurons (LOC) are two types, those that terminate in discrete, dense arbors and those that travel long distances making contacts in passing and giving off terminal branches en route. In this project it is proposed to examine the differences in structure and organization between the two kinds of LOC, Shell and Intristic neurons. Relying principally on axonal tracing methods involving DiI, a lipophilic dye that can be used with fixed material, 1) the tonotopic organization of the efferent innervation of the cochlea provided by intrinsic and Shell neurons will be determined in the rat, 2) the prevalence of branching of efferent axons belonging to Intrinsic and Shell neurons will be determined in the rat, since profuse branching could well amplify the impact of a relatively small number of neurons, 3) the specific structures that are contacted by Shell axons in the rat will be determined using the electron microscope, and finally, 4) in order to assess the biological generality of dual LOC systems, attempts will be made to identify Shell and Intrinsic axons in several species in which preliminary evidence suggests dual LOC efferents may exist, including the mouse, gerbil, chinchilla and cat. The studies in the mouse and cat are expected to provide important normative data on the efferent innervation in species being used in Projects II and III, respectively.